Xbar devices with excess piezoelectric material removed

ABSTRACT

A filter device has a substrate with a first cavity and a second cavity on a single die; and a bonding layer formed on the substrate but not spanning the first cavity or the second cavity. A piezoelectric plate is bonded to the bonding layer and spans the first and the second cavity. However, excess portions of piezoelectric plate are removed that extend a certain length past the perimeter of the first cavity and of the second cavity. Excess portions may be piezoelectric material that extends in the length and width direction past the perimeter of a cavity by more than between 2 and 25 percent of the cavity perimeter. An interdigital transducer (IDT) is on a front surface of the piezoelectric plate and having interleaved fingers over the first cavity.

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

RELATED APPLICATION INFORMATION

This patent is a continuation of patent application Ser. No. 17/123,029,titled XBAR DEVICES WITH EXCESS PIEZOELECTRIC MATERIAL REMOVED, filedDec. 15, 2020, which claims priority to U.S. provisional patentapplication No. 63/113,301, filed Nov. 13, 2020, which is incorporatedhere by reference.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic waveresonators, and specifically to filters for use in communicationsequipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a passband or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is better than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of cellular base stations, mobile telephone andcomputing devices, satellite transceivers and ground stations, IoT(Internet of Things) devices, laptop computers and tablets, fixed pointradio links, and other communications systems. RF filters are also usedin radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave (BAW) resonators, film bulkacoustic wave resonators (FBAR), and other types of acoustic resonators.However, these existing technologies are not well-suited for use at thehigher frequencies and bandwidths proposed for future communicationsnetworks.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. Radio accesstechnology for mobile telephone networks has been standardized by the3GPP (3^(rd) Generation Partnership Project). Radio access technologyfor 5^(th) generation mobile networks is defined in the 5G NR (newradio) standard. The 5G NR standard defines several new communicationsbands. Two of these new communications bands are n77, which uses thefrequency range from 3300 MHz to 4200 MHz, and n79, which uses thefrequency range from 4400 MHz to 5000 MHz. Both band n77 and band n79use time-division duplexing (TDD), such that a communications deviceoperating in band n77 and/or band n79 use the same frequencies for bothuplink and downlink transmissions. Bandpass filters for bands n77 andn79 must be capable of handling the transmit power of the communicationsdevice. WiFi bands at 5 GHz and 6 GHz also require high frequency andwide bandwidth. The 5G NR standard also defines millimeter wavecommunication bands with frequencies between 24.25 GHz and 40 GHz.

The Transversely-Excited Film Bulk Acoustic Resonator (XBAR) is anacoustic resonator structure for use in microwave filters. The XBAR isdescribed in U.S. Pat. No. 10,491,291, titled TRANSVERSELY EXCITED FILMBULK ACOUSTIC RESONATOR. An XBAR resonator comprises an interdigitaltransducer (IDT) formed on a thin floating layer, or diaphragm, of asingle-crystal piezoelectric material. The IDT includes a first set ofparallel fingers, extending from a first busbar and a second set ofparallel fingers extending from a second busbar. The first and secondsets of parallel fingers are interleaved. A microwave signal applied tothe IDT excites a shear primary acoustic wave in the piezoelectricdiaphragm. XBAR resonators provide very high electromechanical couplingand high frequency capability. XBAR resonators may be used in a varietyof RF filters including band-reject filters, band-pass filters,duplexers, and multiplexers. XBARs are well suited for use in filtersfor communications bands with frequencies above 3 GHz.

DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic plan view and two schematic cross-sectionalviews of a transversely-excited film bulk acoustic resonator (XBAR).

FIG. 2 is an expanded schematic cross-sectional view of a portion of theXBAR of FIG. 1 .

FIG. 3A is an alternative schematic cross-sectional view of an XBAR.

FIG. 3B is a graphical illustration of the primary acoustic mode ofinterest in an XBAR.

FIG. 4A is a cross-sectional view of a simulation of the acoustic modesexcited between conductors in an XBAR device.

FIG. 4B is a graph of the conductance of the structure shown in FIG. 4A

FIG. 5A is a schematic circuit diagram and layout for a high frequencyband-pass filter using XBARs.

FIG. 5B is a schematic plan view of a filter incorporating five XBARdevices.

FIG. 5C is a schematic cross-sectional view at the plane B-B defined inDetail A of FIG. 5B.

FIGS. 5D, 5E, 5F, and 5G are schematic cross-sectional views at theplane C-C defined in FIG. 5B.

FIG. 6 is a flow chart showing a process for making an XBAR havingexcess piezoelectric material removed.

FIG. 7 is a schematic cross-sectional view of an XBAR resonator at theplane B-B defined in Detail A of FIG. 5B prior to removing excesspiezoelectric material.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator orthe same two least significant digits.

DETAILED DESCRIPTION

Description of Apparatus

The Shear-Mode Film Bulk Acoustic Resonator (XBAR) is a new resonatorstructure for use in microwave filters. The XBAR is described in U.S.Pat. No. 10,491,291, titled TRANSVERSELY EXCITED FILM BULK ACOUSTICRESONATOR, which is incorporated herein by reference in its entirety. AnXBAR resonator comprises an interdigital transducer (IDT) formed on athin floating layer, membrane or diaphragm, of a piezoelectric material.A microwave signal applied to the IDT excites a shear primary acousticwave in the piezoelectric diaphragm, such that the acoustic energy flowssubstantially normal to the surfaces of the layer, which is orthogonalor transverse to the direction of the electric field generated by theIDT. XBAR resonators provide very high electromechanical coupling andhigh frequency capability.

The following describes improved XBAR resonators, filters andfabrication techniques for XBAR resonators with excess piezoelectricmaterial removed. The excess piezoelectric material between conductors(other than the resonator IDTs) of an RF filter is removed to avoidexciting acoustic modes that then couple to the substrate and increaseinsertion loss. The excess piezoelectric material may be removed frombetween pairs of conductors outside of the XBAR resonators of an RFfilter, such as from between a signal conductor and a ground conductor,or from between two signal conductors.

FIG. 1 shows a simplified schematic top view and orthogonalcross-sectional views of a transversely-excited film bulk acousticresonator (XBAR) 100. XBAR resonators such as the resonator 100 may beused in a variety of RF filters including band-reject filters, band-passfilters, duplexers, and multiplexers. XBARs are particularly suited foruse in filters for communications bands with frequencies above 3 GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on asurface of a piezoelectric plate 110 having parallel front and backsurfaces 112, 114, respectively. The piezoelectric plate is a thinsingle-crystal layer of a piezoelectric material such as lithiumniobate, lithium tantalate, lanthanum gallium silicate, gallium nitride,or aluminum nitride. The piezoelectric plate is cut such that theorientation of the X, Y, and Z crystalline axes with respect to thefront and back surfaces is known and consistent. In the examplespresented, the piezoelectric plates may be Z-cut, which is to say the Zaxis is normal to the surfaces. However, XBARs may be fabricated onpiezoelectric plates with other crystallographic orientations.

The back surface 114 of the piezoelectric plate 110 is attached to asubstrate 120 that provides mechanical support to the piezoelectricplate 110. The substrate 120 may be, for example, silicon, sapphire,quartz, or some other material. The substrate may have layers of siliconthermal oxide (TOX) and crystalline silicon. The back surface 114 of thepiezoelectric plate 110 may be bonded to the substrate 120 using a waferbonding process, or grown on the substrate 120, or attached to thesubstrate in some other manner. The piezoelectric plate may be attacheddirectly to the substrate or may be attached to the substrate via one ormore intermediate material layers. As shown in FIG. 1 , the diaphragm115 is contiguous with the rest of the piezoelectric plate 110 aroundall of a perimeter 145 of the cavity 140. In this context, “contiguous”means “continuously connected without any intervening item”.

The conductor pattern of the XBAR 100 includes an interdigitaltransducer (IDT) 130. The IDT 130 includes a first plurality of parallelfingers, such as finger 136, extending from a first busbar 132 and asecond plurality of fingers extending from a second busbar 134. Thefirst and second pluralities of parallel fingers are interleaved. Theinterleaved fingers 136 overlap for a distance AP, commonly referred toas the “aperture” of the IDT. The center-to-center distance L betweenthe outermost fingers of the IDT 130 is the “length” of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR100. A radio frequency or microwave signal applied between the twobusbars 132, 134 of the IDT 130 excites a primary acoustic mode withinthe piezoelectric plate 110. As will be discussed in further detail, theexcited primary acoustic mode is a bulk shear mode where acoustic energypropagates along a direction substantially orthogonal to the surface ofthe piezoelectric plate 110, which is also normal, or transverse, to thedirection of the electric field created by the IDT fingers. Thus, theXBAR is considered a transversely-excited film bulk wave resonator.

A cavity 140 is formed in the substrate 120 such that a portion 115 ofthe piezoelectric plate 110 containing the IDT 130 is suspended over thecavity 140 without contacting the substrate 120 or the bottom of thecavity. “Cavity” has its conventional meaning of “an empty space withina solid body.” The cavity may contain a gas, air, or a vacuum. In somecase, there is also a second substrate, package or other material havinga cavity (not shown) above the plate 110, which may be a mirror image ofsubstrate 120 and cavity 140. The cavity above plate 110 may have anempty space depth greater than that of cavity 140. The fingers extendover (and part of the busbars may optionally extend over) the cavity (orbetween the cavities). The cavity 140 may be a hole completely throughthe substrate 120 (as shown in Section A-A and Section B-B of FIG. 1 )or a recess in the substrate 120 (as shown subsequently in FIG. 3A). Thecavity 140 may be formed, for example, by selective etching of thesubstrate 120 before or after the piezoelectric plate 110 and thesubstrate 120 are attached. As shown in FIG. 1 , the cavity 140 has arectangular shape with an extent greater than the aperture AP and lengthL of the IDT 130. A cavity of an XBAR may have a different shape, suchas a regular or irregular polygon. The cavity of an XBAR may more orfewer than four sides, which may be straight or curved.

The portion 115 of the piezoelectric plate suspended over the cavity 140will be referred to herein as the “diaphragm” (for lack of a betterterm) due to its physical resemblance to the diaphragm of a microphone.The diaphragm may be continuously and seamlessly connected to the restof the piezoelectric plate 110 around all, or nearly all, of perimeterof the cavity 140. In this context, “contiguous” means “continuouslyconnected without any intervening item”.

For ease of presentation in FIG. 1 , the geometric pitch and width ofthe IDT fingers is greatly exaggerated with respect to the length(dimension L) and aperture (dimension AP) of the XBAR. A typical XBARhas more than ten parallel fingers in the IDT 110. An XBAR may havehundreds, possibly thousands, of parallel fingers in the IDT 110.Similarly, the thickness of the fingers in the cross-sectional views isgreatly exaggerated.

FIG. 2 shows a detailed schematic cross-sectional view of the XBAR 100of FIG. 1 . The cross-sectional view may be a portion of the XBAR 100that includes fingers of the IDT. The piezoelectric plate 110 is asingle-crystal layer of piezoelectrical material having a thickness ts.The ts may be, for example, 100 nm to 1500 nm. When used in filters forLTE™ bands from 3.4 GHZ to 6 GHz (e.g. bands 42, 43, 46), the thicknessts may be, for example, 200 nm to 1000 nm.

A front-side dielectric layer 214 may optionally be formed on the frontside of the piezoelectric plate 110. The “front side” of the XBAR is, bydefinition, the surface facing away from the substrate. The front-sidedielectric layer 214 has a thickness tfd. The front-side dielectriclayer 214 is formed between the IDT fingers 238. Although not shown inFIG. 2 , the front side dielectric layer 214 may also be deposited overthe IDT fingers 238. A back-side dielectric layer 216 may optionally beformed on the back side of the piezoelectric plate 110. The back-sidedielectric layer 216 has a thickness tbd. The front-side and back-sidedielectric layers 214, 216 may be a non-piezoelectric dielectricmaterial, such as silicon dioxide or silicon nitride. The tfd and tbdmay be, for example, 0 to 500 nm. tfd and tbd are typically less thanthe thickness ts of the piezoelectric plate. The tfd and tbd are notnecessarily equal, and the front-side and back-side dielectric layers214, 216 are not necessarily the same material. Either or both of thefront-side and back-side dielectric layers 214, 216 may be formed ofmultiple layers of two or more materials.

The front side dielectric layer 214 may be formed over the IDTs of some(e.g., selected ones) of the XBAR devices in a filter. The front sidedielectric 214 may be formed between and cover the IDT finger of someXBAR devices but not be formed on other XBAR devices. For example, afront side frequency-setting dielectric layer may be formed over theIDTs of shunt resonators to lower the resonance frequencies of the shuntresonators with respect to the resonance frequencies of seriesresonators, which have thinner or no front side dielectric. Some filtersmay include two or more different thicknesses of front side dielectricover various resonators. The resonance frequency of the resonators canbe set thus “tuning” the resonator, at least in part, by selecting athicknesses of the front side dielectric.

Further, a passivation layer may be formed over the entire surface ofthe XBAR device 100 except for contact pads where electric connectionsare made to circuitry external to the XBAR device. The passivation layeris a thin dielectric layer intended to seal and protect the surfaces ofthe XBAR device while the XBAR device is incorporated into a package.The front side dielectric layer and/or the passivation layer may be,SiO₂, Si₃N₄, Al₂O₃, some other dielectric material, or a combination ofthese materials.

The thickness of the passivation layer may be selected to protect thepiezoelectric plate and the metal conductors from water and chemicalcorrosion, particularly for power durability purposes. It may range from10 to 100 nm. The passivation material may consist of multiple oxideand/or nitride coatings such as SiO₂ and Si₃N₄ material.

The IDT fingers 238 may be one or more layers of aluminum or asubstantially aluminum alloy, copper or a substantially copper alloy,beryllium, tungsten, molybdenum, gold, or some other conductivematerial. Thin (relative to the total thickness of the conductors)layers of other metals, such as chromium or titanium, may be formedunder and/or over the fingers to improve adhesion between the fingersand the piezoelectric plate 110 and/or to passivate or encapsulate thefingers. The busbars (132, 134 in FIG. 1 ) of the IDT may be made of thesame or different materials as the fingers.

Dimension p is the center-to-center spacing or “pitch” of the IDTfingers, which may be referred to as the pitch of the IDT and/or thepitch of the XBAR. Dimension w is the width or “mark” of the IDTfingers. The IDT of an XBAR differs substantially from the IDTs used insurface acoustic wave (SAW) resonators. In a SAW resonator, the pitch ofthe IDT is one-half of the acoustic wavelength at the resonancefrequency. Additionally, the mark-to-pitch ratio of a SAW resonator IDTis typically close to 0.5 (i.e. the mark or finger width is aboutone-fourth of the acoustic wavelength at resonance). In an XBAR, thepitch p of the IDT is typically 2 to 20 times the width w of thefingers. In addition, the pitch p of the IDT is typically 2 to 20 timesthe thickness is of the piezoelectric slab 212. The width of the IDTfingers in an XBAR is not constrained to one-fourth of the acousticwavelength at resonance. For example, the width of XBAR IDT fingers maybe 500 nm or greater, such that the IDT can be fabricated using opticallithography. The thickness tm of the IDT fingers may be from 100 nm toabout equal to the width w. The thickness of the busbars (132, 134 inFIG. 1 ) of the IDT may be the same as, or greater than, the thicknesstm of the IDT fingers.

FIG. 3A is an alternative cross-sectional view of XBAR device 300 alongthe section plane A-A defined in FIG. 1 . In FIG. 3A, a piezoelectricplate 310 is attached to a substrate 320. A portion of the piezoelectricplate 310 forms a diaphragm 315 spanning a cavity 340 in the substrate.The cavity 340, does not fully penetrate the substrate 320, and isformed in the substrate under the portion of the piezoelectric plate 310containing the IDT of an XBAR. Fingers, such as finger 336, of an IDTare disposed on the diaphragm 315. Plate 310, diaphragm 315 and fingers336 may be plate 110, diaphragm 115 and fingers 136. The cavity 340 maybe formed, for example, by etching the substrate 320 before attachingthe piezoelectric plate 310. Alternatively, the cavity 340 may be formedby etching the substrate 320 with a selective etchant that reaches thesubstrate through one or more openings 342 provided in the piezoelectricplate 310. The diaphragm 315 may be contiguous with the rest of thepiezoelectric plate 310 around a large portion of a perimeter 345 of thecavity 340. For example, the diaphragm 315 may be contiguous with therest of the piezoelectric plate 310 around at least 50% of the perimeterof the cavity 340.

One or more intermediate material layers 322 may be attached betweenplate 310 and substrate 320. An intermediary layer may be a bondinglayer, an etch stop layer, a sealing layer, an adhesive layer or layerof other material that is attached or bonded to plate 310 and substrate320. In other embodiments, the piezoelectric plate 310 is attacheddirectly to the substrate 320 and an intermediary layer does not exist.

While the cavity 340 is shown in cross-section, it should be understoodthat the lateral extent of the cavity is a continuous closed band areaof substrate 320 that surrounds and defines the size of the cavity 340in the direction normal to the plane of the drawing. The lateral (i.e.left-right as shown in the figure) extent of the cavity 340 is definedby the lateral edges substrate 320. The vertical (i.e. down from plate310 as shown in the figure) extent or depth of the cavity 340 intosubstrate 320. In this case, the cavity 340 has a side cross-sectionrectangular, or nearly rectangular, cross section.

The XBAR 300 shown in FIG. 3A will be referred to herein as a“front-side etch” configuration since the cavity 340 is etched from thefront side of the substrate 320 (before or after attaching thepiezoelectric plate 310). The XBAR 100 of FIG. 1 will be referred toherein as a “back-side etch” configuration since the cavity 140 isetched from the back side of the substrate 120 after attaching thepiezoelectric plate 110. The XBAR 300 shows one or more openings 342 inthe piezoelectric plate 310 at the left and right sides of the cavity340. However, in some cases openings 342 in the piezoelectric plate 310are only at the left or right side of the cavity 340.

FIG. 3B is a graphical illustration of the primary acoustic mode ofinterest in an XBAR. FIG. 3B shows a small portion of an XBAR 350including a piezoelectric plate 310 and three interleaved IDT fingers336. XBAR 350 may be part of any XBAR herein. An RF voltage is appliedto the interleaved fingers 336. This voltage creates a time-varyingelectric field between the fingers. The direction of the electric fieldis primarily lateral, or parallel to the surface of the piezoelectricplate 310, as indicated by the arrows labeled “electric field”. Due tothe high dielectric constant of the piezoelectric plate, the electricfield is highly concentrated in the plate relative to the air. Thelateral electric field introduces shear deformation, and thus stronglyexcites a primary shear-mode acoustic mode, in the piezoelectric plate310. In this context, “shear deformation” is defined as deformation inwhich parallel planes in a material remain parallel and maintain aconstant distance while translating relative to each other. A “shearacoustic mode” is defined as an acoustic vibration mode in a medium thatresults in shear deformation of the medium. The shear deformations inthe XBAR 350 are represented by the curves 360, with the adjacent smallarrows providing a schematic indication of the direction and magnitudeof atomic motion. The degree of atomic motion, as well as the thicknessof the piezoelectric plate 310, have been greatly exaggerated for easeof visualization. While the atomic motions are predominantly lateral(i.e. horizontal as shown in FIG. 3B), the direction of acoustic energyflow of the excited primary shear acoustic mode is substantiallyorthogonal to the front and back surface of the piezoelectric plate, asindicated by the arrow 365.

An acoustic resonator based on shear acoustic wave resonances canachieve better performance than current state-of-the artfilm-bulk-acoustic-resonators (FBAR) and solidly-mounted-resonatorbulk-acoustic-wave (SMR BAW) devices where the electric field is appliedin the thickness direction. The piezoelectric coupling for shear waveXBAR resonances can be high (>20%) compared to other acousticresonators. High piezoelectric coupling enables the design andimplementation of microwave and millimeter-wave filters with appreciablebandwidth.

FIG. 4A is a cross-sectional view of a simulation of the acoustic modes460 excited by two conductors 424 and 426 in an XBAR device 400. Thedevice includes a 400 nm thick layer of lithium niobate piezoelectricmaterial 410 bonded to a silicon substrate 420 that is 250 um thick. A 2micron thick silicon dioxide bonding layer 422 is disposed between thepiezoelectric layer 410 and the substrate 420. The bonding layer may beAl2O3 or SiO2. Bonding layer 422 may be bonded to layer 410 andsubstrate 420, thus bonding them together. Two conductors 424 and 426(e.g., electrodes) are formed on the top surface of piezoelectric layer410. The conductors represent, for example, a signal electrode and aground electrode on the surface of an XBAR filter. The conductors may bea certain distance past the perimeter of any cavity of a resonator ofthe device, such as a certain distance noted for FIG. 5B. The electrodesare aluminum, 500 nm thick, and separated by 80 microns. Thepiezoelectric layer 410 and bonding layer 422 extend across the 80micron separation without being bonded to or covered by the conductors.

A radio frequency electric field between the conductors 424 and 426 (aswould occur if 424 and 426 are a signal conductor and a ground conductorin a filter) excites a shear acoustic mode in the piezoelectric layer410 between the conductors. The acoustic mode travels through thebonding layer and the substrate. If the back surface of the substrate ispolished, the acoustic mode reflects such that the surface of thepiezoelectric plate and the back surface of the substrate form aresonant cavity. If the back surface of the substrate is textured (forexample by grinding) the acoustic mode is dispersed after reflection.

FIG. 4B is a graph 470 of the Conductance (in Siemens per meter ofconductor length) of the structure shown in FIG. 4A as a function offrequency (GHz). The curve 471 is a plot of the conductance when theback surface of the silicon substrate 420 is polished. In this case, thetop surface of the piezoelectric layer and the back surface of thesubstrate form a cavity that results in resonance peaks 472 separated byabout 10 MHz. These resonance peaks may result in undesired effects suchas ripple within the passband of the filter having device 400. The curve475 is a plot of the conductance when the back surface of the siliconsubstrate is suitably textured (e.g. fine ground). In this case, theresonance peaks do not form. The residual conductance contributes to theinsertion loss of the filter. This increase in insertion loss can bedetrimental to filter performance.

FIG. 5A is a schematic circuit diagram and layout for a high frequencyband-pass filter 500 using XBARs. The filter 500 has a conventionalladder filter architecture including three series resonators 510A, 510B,510C and two shunt resonators 520A, 520B. The three series resonators510A, 510B, and 510C are connected in series between a first port and asecond port. In FIG. 5A, the first and second ports are labeled “In” and“Out”, respectively. However, the filter 500 is bidirectional and eitherport and serve as the input or output of the filter. The two shuntresonators 520A, 520B are connected from nodes between the seriesresonators to ground. All the shunt resonators and series resonators areXBARs on a single die.

The three series resonators 510A, B, C and the two shunt resonators520A, B of the filter 500 are formed on a single plate 410 ofpiezoelectric material bonded to a silicon substrate (not visible). Eachresonator includes a respective IDT (not shown), with at least thefingers of the IDT disposed over a cavity in the substrate. In this andsimilar contexts, the term “respective” means “relating things each toeach”, which is to say with a one-to-one correspondence. In FIG. 5A, thecavities are illustrated schematically as the dashed rectangles (such asthe rectangle 535). In this example, each IDT is disposed over arespective cavity. In other filters, the IDTs of two or more resonatorsmay be disposed over a single cavity.

In some cases, to produce improved XBAR resonators and filters withexcess piezoelectric material removed, the portions or areas of thepiezoelectric material of plate 410 that extend a certain distance pastthe perimeter 545 of the cavities of filter 500 (or cavity perimeter 135of FIG. 1 ) may be removed. This removing may include removing thepiezoelectric material: a) that extends in the length direction past theperimeter of the cavity by between 2 and 25 percent more the length ofthe cavity; and b) that extends in the width direction past theperimeter of the cavity by between 2 and 25 percent more the width ofthe cavity. This removing may include removing the excess piezoelectricmaterial between conductors (other than the resonator IDTs) of an RFfilter to avoid exciting acoustic modes that then couple to thesubstrate and increase insertion loss. This removing may includeremoving the excess piezoelectric material from between pairs ofconductors outside of the XBAR resonators of an RF filter, such as frombetween a signal conductor and a ground conductor, or from between twosignal conductors.

FIG. 5B is a schematic plan view of a filter 550 incorporating five XBARdevices labeled “X1” to “X5”. The filter 550 is exemplary and does notrepresent any particular application. The filter 550 includes five XBARdevices X1-X5. Filter 550 may be filter 500 of FIG. 5A, where device X1is series resonator 510A, device X2 is shunt resonator 520A, device X3is series resonator 510B, device X4 is shunt resonator 520B, and deviceX5 is series resonator 510C. The filter 550 may be formed on a singledie. A “die” may be a semiconductor chip or integrated circuit (IC) chipthat is diced from other chips such as of a wafer. It may be amonolithic integrated circuit (also referred to as an IC, a chip, or amicrochip) that has a set of electronic circuits on one small flat piece(or “chip”) of semiconductor material that is normally silicon.

The horizontally-hatched areas 552 represent the IDT and/or fingers ofthe XBAR devices. The fingers of the IDTs are not to scale. FIG. 5Bshows a ground (GND) conductor of the filter 550 which may be connectedto or part of busbars on one side of the XBARs X2 and X4 as shown. TheGND conductor is connected to a ground signal of the filter 550. FIG. 5Bshows an input (IN) signal conductor of the filter 550 which may beconnected to or part of busbars on one side of the XBAR X1 as shown. TheIN conductor is connected to an input signal for the filter 550. FIG. 5Bshows an output (OUT) signal conductor of the filter 550 which may beconnected to or part of busbars on one side of the XBAR X5 as shown. TheOUT conductor is connected to an output signal for the filter 550.

FIG. 5B and detail A show the dashed lines outline of the cavityperimeters, such as perimeter 585 behind the IDT fingers. Perimeter 585may represent a cavity perimeter such as perimeter 135 or 535. FIG. 5Band detail A also show the dot-dash lines perimeter of the piezoelectricmaterial perimeters, such as perimeter 590. Perimeter 590 may representa perimeter of the piezoelectric material that: a) extends in thepiezoelectric material length direction LP past the perimeter of thecavity length LC by more than 5, 10 or 20 percent of the length of thecavity LC; and b) that extends in the piezoelectric material widthdirection WP past the perimeter of the perimeter of the cavity width WCby more than 5, 10 or 20 percent of a width of the cavity WC. This maybe true for any one or more (up to all) of the five XBAR devices X1-X5.

The piezoelectric material may be removed from the entire surface of thefilter 550 except within the rectangles defined by the dot-dash lines,such as perimeter 590 and the similar perimeters of XBAR devices X1-X4.The outlines of the cavities and the piezoelectric layer are shown asrectangles for ease of presentation but may have other shapes. Forexample, the perimeters of the cavities and piezoelectric layers may beirregular polygons or generally rectangular with non-straight (e.g.curved, serrated, or wavy) sides.

FIG. 5C is a schematic cross-sectional 595 view at the plane B-B definedin Detail A of FIG. 5B. FIG. 5C shows filter device X5 comprisingsubstrate 520 having cavity 540. The substrate has additional cavitieswhere devices X1-X4 are formed and may be a single die. Bonding layer522 is formed on the substrate but is not over the cavity 540.Piezoelectric plate 510 is bonded to the bonding layer 522 and spans thecavity 540. In some cases, layer 522 does not exist and the plate isdirectly attached to the substrate. An interdigital transducer (IDT)formed on a front surface of the piezoelectric plate 510 has interleavedfingers 536 over the cavity 540. Although the conductors are shown asmetal, they may be another proper conductive material. Although thesubstrate is shown as silicon, it may be another proper substratematerial. Although the bonding layer is shown as silicon dioxide, it maybe another proper bonding material.

The Piezoelectric plate 510 includes the diaphragm having piezoelectricmaterial spanning the cavity and excess portions that extend a certainlength past the perimeter of the cavity. The excess portions may extenda certain length and width distance (LP and WP) past the length andwidth perimeter of the cavity (LC and WC). The excess portions may be aperimeter of the piezoelectric material that extends in the length andwidth direction past the perimeter of the cavity by: a) more than 5, 10or 20 percent; or b) between 2 and 25 percent.

Fingers 536 may span or be over cavity 540. In some cases, part of thebusbars of the IDT is also over the cavity. In other cases, all of thebusbars are over the substrate 520 but not over the cavity. At leastportions of the busbars are over the substrate (e.g., not over thecavity) to better conduct heat generated in the IDT to the substrate.

The thicknesses of the piezoelectric layer 510, bonding layer 522,fingers 536, and metal conductors 524 and 526 are greatly exaggeratedfor ease of depiction. The left side of FIG. 5C illustrates the casewhere the piezoelectric layer 510, but not the SiO₂ bonding layer 522,is removed outside of the area of the resonator cavity 540, such asremoved from extending beyond width WP. The right side of FIG. 5Cillustrates the case where both the piezoelectric layer 510 and thebonding layer 522 are removed outside of the area of the resonatorcavity 540, such as removed from extending beyond width WP. Thisrightside configuration provides an improved thermal connection betweenthe metal conductor 524 and the Si substrate 520, but requires the metalconductor cover a larger height step 598 than on the left side.

FIGS. 5D, 5E, 5F, and 5G are schematic cross-sectional views at theplane C-C defined in FIG. 5B. These views show a cross-section though aconductor 524/526 remote from a resonator, such as noted for FIGS.4A-5C. FIG. 5D and FIG. 5E are consistent with the right and left sidesof FIG. 5C, respectively. FIG. 5F is an alternative configuration inwhich the excess piezoelectric material 510 is removed after theconductor patterns 524 and 526 are formed. In this case, thepiezoelectric layer 510 and the SiO₂ bonding layer 522 remain beneaththe conductor. This configuration eliminates acoustic losses withoutrequiring the conductors to cover steps 598 in the underlying layers.FIG. 5G extends the configuration of 5F by removing a portion of the Sisubstrate 520 between conductors 524 and 526 to reduce straycapacitance.

FIGS. 5B, 5C, 5D, and 5E illustrate a conceptually easy solution to theproblems of FIGS. 4A-B, which is to etch away the undesired portions ofthe piezoelectric plate 510 immediately after bonding the plate 510 tothe substrate 520 or bonding layer 522 (e.g., at 625A in FIG. 6 ). FIGS.5F and 5G illustrate an alternative process sequence where the undesiredportions of the piezoelectric plate 510 are etched (e.g., at 625B inFIG. 6 ) after the conductors 524 and 526 are formed. The benefit of thealternative process is that the conductors do not have to go over stepswhere the piezoelectric plate has been removed, such as shown at step598 of FIG. 5C. The conductor thickness is typically 500 nm and thepiezoelectric plate thickness is typically 400 nm which may cause aconductor bonding problem at or near the step, such as gaps between theconductor and bonding layer or substrate. These step may also causeother fabrication problems.

One problem being solved by removing the portions of the piezoelectricmaterial that extend a certain distance LP and WP past the perimeter ofthe cavity LC and WC of an XBAR resonator is caused by piezoelectricmaterial between conductors, such as in the 80 um gap between conductors424 and 426 as noted for FIGS. 4A-B. Piezoelectric material under theconductors such as under conductors 424 and 426 as noted for FIGS. 4A-B,or for FIGS. 5D-G does not excite acoustic modes.

Description of Methods

FIG. 6 is a simplified flow chart showing a process 600 for making anXBAR having excess piezoelectric material removed or a filterincorporating XBARs having excess piezoelectric material removed. Thisis the same as the process defined in pending application Ser. No.16/438,121, which is incorporated herein by reference, with the addedstep of removing the excess piezoelectric material at 625A before theconductor patterns are formed or at 625B after the conductor patternsare formed. The process 600 starts at 605 with a substrate and a plateof piezoelectric material and ends at 695 with a completed XBAR orfilter having excess piezoelectric material removed, such as shown forFIGS. 5A-5G. As will be described subsequently, the piezoelectric platemay be mounted on a sacrificial substrate or may be a portion of waferof piezoelectric material. The flow chart of FIG. 6 includes only majorprocess steps. Various conventional process steps (e.g. surfacepreparation, chemical mechanical processing (CMP), cleaning, inspection,deposition, photolithography, baking, annealing, monitoring, testing,etc.) may be performed before, between, after, and during the stepsshown in FIG. 6 .

The flow chart of FIG. 6 captures three variations of the process 600for making an XBAR which differ in when and how cavities are formed inthe substrate. The cavities may be formed at steps 610A, 610B, or 610C.Only one of these steps is performed in each of the three variations ofthe process 600.

The flow chart of FIG. 6 also captures two variations of the process 600for making an XBAR which differ in when and how excess piezoelectricmaterial is removed. The excess piezoelectric material may be removed atstep 625A or 625B. Only one of these steps is performed in each of thesetwo variations of the process 600. In another variation, some of theexcess piezoelectric material may be removed at step 625A and more if itremoved at step 625B.

The piezoelectric plate may be, for example, Z-cut, rotated Z-cut, orrotated Y-cut lithium niobate or lithium tantalate or a material notedfor plate 110. The piezoelectric plate may be some other material and/orsome other cut. The substrate may be silicon. The substrate may be someother material that allows formation of deep cavities by etching orother processing. The silicon substrate may have layers of silicon TOXand polycrystalline silicon.

In one variation of the process 600, one or more cavities are formed inthe substrate at 610A, before the piezoelectric plate is bonded to thesubstrate at 620. A separate cavity may be formed for each resonator ina filter device. The one or more cavities may be formed usingconventional photolithographic and etching techniques. These techniquesmay be isotropic or anisotropic; and may use deep reactive ion etching(DRIE). Typically, the cavities formed at 610A will not penetratethrough the substrate, and the resulting resonator devices will have across-section as shown in FIG. 3A.

At 620, the piezoelectric plate is bonded to the substrate. Thepiezoelectric plate and the substrate may be bonded by a wafer bondingprocess. Typically, the mating surfaces of the substrate and thepiezoelectric plate are highly polished. One or more layers ofintermediate materials, such as an oxide or metal, may be formed ordeposited on the mating surface of one or both of the piezoelectricplate and the substrate. One or both mating surfaces may be activatedusing, for example, a plasma process. The mating surfaces may then bepressed together with considerable force to establish molecular bondsbetween the piezoelectric plate and the substrate or intermediatematerial layers. In some cases, bonding layer 522 may be used to bondthe plate to the substrate.

In a first variation of 620, the piezoelectric plate is initiallymounted on a sacrificial substrate. After the piezoelectric plate andthe substrate are bonded, the sacrificial substrate, and any interveninglayers, are removed to expose the surface of the piezoelectric plate(the surface that previously faced the sacrificial substrate). Thesacrificial substrate may be removed, for example, by material-dependentwet or dry etching or some other process.

In a second variation of 620 starts with a single-crystal piezoelectricwafer. Ions are implanted to a controlled depth beneath a surface of thepiezoelectric wafer (not shown in FIG. 6 ). The portion of the waferfrom the surface to the depth of the ion implantation is (or willbecome) the thin piezoelectric plate and the balance of the wafer iseffectively the sacrificial substrate. After the implanted surface ofthe piezoelectric wafer and device substrate are bonded, thepiezoelectric wafer may be split at the plane of the implanted ions (forexample, using thermal shock), leaving a thin plate of piezoelectricmaterial exposed and bonded to the substrate. The thickness of the thinplate piezoelectric material is determined by the energy (and thusdepth) of the implanted ions. The process of ion implantation andsubsequent separation of a thin plate is commonly referred to as “ionslicing”. The exposed surface of the thin piezoelectric plate may bepolished or planarized after the piezoelectric wafer is split.

In one variation of the process 600, at 625A the portions of thepiezoelectric material that extend a certain distance past the perimeterof the cavity of the XBAR resonator are removed after the piezoelectricplate is bonded to the substrate at 620 and before the conductor patternis formed at 630. This may be removing piezoelectric material extendingbeyond LP and WP of the resonator. The portions may be removed bypatterning and etching to remove the piezoelectric material that extendsa certain distance past the perimeter of the cavity. Removing theportions of piezoelectric material may include removing bonding layer522 that is below the excess portions of the piezoelectric layer thatare removed. In other cases, it does not and those portions of layer 522remain. Here, bonding layer 522 can be used as an etch stop for removingthe excess portions piezoelectric material.

Removing the portions of piezoelectric material may include removing thepiezoelectric material: a) that extends in the length direction past theperimeter of the cavity by more than between 2 and 25 percent the lengthof the cavity; and b) that extends in the width direction past theperimeter of the cavity by more than between 2 and 25 percent the widthof the cavity. This removing may include removing the excesspiezoelectric material between conductors (other than the resonatorIDTs) of an RF filter to avoid exciting acoustic modes that then coupleto the substrate and increase insertion loss. This removing may includeremoving the excess piezoelectric material from between pairs ofconductors outside of the XBAR resonators of an RF filter, such as frombetween a signal conductor and a ground conductor, or from between twosignal conductors.

Removing the portions of piezoelectric material may only be removingpiezoelectric material between conductors, such as in the 80 um gapbetween conductors 424 and 426 as noted for FIGS. 4A-B. Piezoelectricmaterial under the conductors such as under conductors 424 and 426 asnoted for FIGS. 4A-B, or for FIGS. 5D-G is not removed.

FIG. 7 is a schematic cross-sectional view at the plane B-B defined inFIG. 5B of an XBAR resonator 700 prior to removing excess piezoelectricmaterial. This view illustrates the case where the piezoelectric layer710 has excess portions P1 and P2 to be removed from outside of the areaof the resonator cavity 540, such as removed from extending beyond widthWP and length LP (not shown). The excess portions P1 and P2 of layer 710can be removed with or without removing the bonding layer 522 from thoseportions. The portions P1 and P2 may be removed by patterning andetching layer 710. Removing portions P1 and P2 may include removingbonding layer 522 below portions P1 and P2, such as noted at 625A and625B; and/or may include removing the conductor pattern above portionsP1 and P2, such as noted at step 625B. After portions P1 and P2 areremoved, resonator 700 may be further processed to become an XBAR havingexcess piezoelectric material removed as noted herein, such as for FIGS.5C-5G.

Conductor patterns and dielectric layers defining one or more XBARdevices are formed on the surface of the piezoelectric plate at 630.Typically, a filter device will have two or more conductor layers thatare sequentially deposited and patterned. The conductor layers mayinclude bonding pads, gold or solder bumps, or other means for makingconnection between the device and external circuitry. The conductorlayers may be, for example, aluminum, an aluminum alloy, copper, acopper alloy, molybdenum, tungsten, beryllium, gold, or some otherconductive metal. Optionally, one or more layers of other materials maybe disposed below (i.e. between the conductor layer and thepiezoelectric plate) and/or on top of the conductor layer. For example,a thin film of titanium, chrome, or other metal may be used to improvethe adhesion between the conductor layers and the piezoelectric plate.The conductor layers may include bonding pads, gold or solder bumps, orother means for making connection between the device and externalcircuitry.

Conductor patterns may be formed at 630 by depositing the conductorlayers over the surface of the piezoelectric plate and removing excessmetal by etching through patterned photoresist. Alternatively, theconductor patterns may be formed at 630 using a lift-off process.Photoresist may be deposited over the piezoelectric plate and patternedto define the conductor pattern. The conductor layer may be deposited insequence over the surface of the piezoelectric plate. The photoresistmay then be removed, which removes the excess material, leaving theconductor pattern. In some cases, forming at 630 occurs prior to bondingat 620, such as where the IDTs are formed prior to bonding the plate tothe substrate.

In another variation of the process 600, at 625B the portions of thepiezoelectric material that extend a certain distance past the perimeterof the cavity of the XBAR resonator are removed after the conductorpattern is formed at 630 and before front side dielectric is optionallyformed at 640. This may be removing piezoelectric material as noted atstep 625A and/or FIG. 7 . The portions may be removed by patterning andetching to remove the piezoelectric material that extends a certaindistance past the perimeter of the cavity.

Removing this piezoelectric material may include removing conductorpattern that is above the excess portions of the piezoelectric layerthat are removed. Removing the portions of piezoelectric material mayinclude removing bonding layer 522 that is below the excess portions ofthe piezoelectric layer that are removed. In other cases, it does notand those portions of layer 522 remain. Here, bonding layer 522 can beused as an etch stop for removing the excess portions piezoelectricmaterial.

Removing the portions of piezoelectric material may include removingpiezoelectric material beside the IDTs or where the IDTs are not formed.It may include removing piezoelectric material beside or where theconductor material and conductors are not formed.

At 640, a front-side dielectric layer or layers may be formed bydepositing one or more layers of dielectric material on the front sideof the piezoelectric plate, over one or more desired conductor patternsof IDT or XBAR devices. The one or more dielectric layers may bedeposited using a conventional deposition technique such as sputtering,evaporation, or chemical vapor deposition. The one or more dielectriclayers may be deposited over the entire surface of the piezoelectricplate, including on top of the conductor pattern. Alternatively, one ormore lithography processes (using photomasks) may be used to limit thedeposition of the dielectric layers to selected areas of thepiezoelectric plate, such as only between the interleaved fingers of theIDTs. Masks may also be used to allow deposition of differentthicknesses of dielectric materials on different portions of thepiezoelectric plate. In some cases, depositing at 640 includesdepositing a first thickness of at least one dielectric layer over thefront-side surface of selected IDTs, but no dielectric or a secondthickness less than the first thickness of at least one dielectric overthe other IDTs. An alternative is where these dielectric layers are onlybetween the interleaved fingers of the IDTs.

The one or more dielectric layers may include, for example, a dielectriclayer selectively formed over the IDTs of shunt resonators to shift theresonance frequency of the shunt resonators relative to the resonancefrequency of series resonators as described in U.S. Pat. No. 10,491,192.The one or more dielectric layers may include anencapsulation/passivation layer deposited over all or a substantialportion of the device.

The different thickness of these dielectric layers causes the selectedXBARs to be tuned to different frequencies as compared to the otherXBARs. For example, the resonance frequencies of the XBARs in a filtermay be tuned using different front-side dielectric layer thickness onsome XBARs.

As compared to the admittance of an XBAR with tfd=0 (i.e. an XBARwithout dielectric layers), the admittance of an XBAR with tfd=30 nmdielectric layer reduces the resonant frequency by about 145 MHzcompared to the XBAR without dielectric layers. The admittance of anXBAR with tfd=60 nm dielectric layer reduces the resonant frequency byabout 305 MHz compared to the XBAR without dielectric layers. Theadmittance of an XBAR with tfd=90 nm dielectric layer reduces theresonant frequency by about 475 MHz compared to the XBAR withoutdielectric layers. Importantly, the presence of the dielectric layers ofvarious thicknesses has little or no effect on the piezoelectriccoupling.

In a second variation of the process 600, one or more cavities areformed in the back side of the substrate at 610B after all the conductorpatterns and dielectric layers are formed at 630. A separate cavity maybe formed for each resonator in a filter device. The one or morecavities may be formed using an anisotropic or orientation-dependent dryor wet etch to open holes through the back-side of the substrate to thepiezoelectric plate. In this case, the resulting resonator devices willhave a cross-section as shown in FIG. 1 .

In a third variation of the process 600, one or more cavities in theform of recesses in the substrate top layer 322 may be formed at 610C byetching a sacrificial layer formed in the front side of the substrateusing an etchant introduced through openings in the piezoelectric plate.A separate cavity may be formed for each resonator in a filter device.The one or more cavities may be formed using an isotropic ororientation-independent dry etch that passes through holes in thepiezoelectric plate and etches the sacrificial layer formed in recessesin the front-side of the substrate. The one or more cavities formed at610C will not penetrate completely through the substrate top layer 322,and the resulting resonator devices will have a cross-section as shownin FIG. 3A.

In all variations of the process 600, the filter or XBAR device iscompleted at 660. Actions that may occur at 660 include depositing anencapsulation/passivation layer such as SiO₂ or Si₃O₄ over all or aportion of the device; forming bonding pads or solder bumps or othermeans for making connection between the device and external circuitry;excising individual devices from a wafer containing multiple devices;other packaging steps; and testing. Another action that may occur at 660is to tune the resonant frequencies of the resonators within a filterdevice by adding or removing metal or dielectric material from the frontside of the device. After the filter device is completed, the processends at 695. FIGS. 1-3B and 5B-5C may show examples of the fingers ofselected IDTs after completion at 660.

Forming the cavities at 610A may require the fewest total process stepsbut has the disadvantage that the XBAR diaphragms will be unsupportedduring all of the subsequent process steps. This may lead to damage to,or unacceptable distortion of, the diaphragms during subsequentprocessing.

Forming the cavities using a back-side etch at 610B requires additionalhandling inherent in two-sided wafer processing. Forming the cavitiesfrom the back side also greatly complicates packaging the XBAR devicessince both the front side and the back side of the device must be sealedby the package.

Forming the cavities by etching from the front side at 610C does notrequire two-sided wafer processing and has the advantage that the XBARdiaphragms are supported during all of the preceding process steps.However, an etching process capable of forming the cavities throughopenings in the piezoelectric plate will necessarily be isotropic.However, such an etching process using a sacrificial material allows fora controlled etching of the cavity, both laterally (i.e. parallel to thesurface of the substrate) as well as normal to the surface of thesubstrate.

Closing Comments

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

1. A filter device comprising: a substrate having at least a firstcavity and a second cavity on a single die; a piezoelectric plate bondedto the substrate and spanning the first cavity and the second cavity;wherein the piezoelectric plate is not provided in excess portions thatextend a certain length past perimeters of the first cavity and of thesecond cavity; and a first interdigital transducer (IDT) on a frontsurface of the piezoelectric plate and having interleaved fingers overthe first cavity, wherein the first cavity has a first perimeter and thesecond cavity has a second perimeter; and wherein the excess portionsare: a length and width of a perimeter of the piezoelectric materialspanning the first cavity and the second cavity that extends more thanbetween 2 and 25 percent past a length and width of the perimeter of thefirst and second cavity, respectively.
 2. The filter device of claim 1,wherein a length and width of a perimeter of the piezoelectric materialspanning the first cavity and the second cavity that extends more than 5percent past a length and width of the perimeter of the first and secondcavity, respectively
 3. The filter device of claim 1, furthercomprising: a bonding layer formed on the substrate but not spanning thefirst cavity or the second cavity; wherein the piezoelectric plate isbonded to the bonding layer; wherein the bonding layer is not providedin excess portions that extend the certain length past the perimeter ofthe first cavity and of the second cavity.
 4. The filter device of claim1, wherein the substrate is Si, the bonding layer is SiO2 and the firstIDT is metal.
 5. The filter device of claim 4, wherein the piezoelectricplate is one of lithium niobate or lithium tantalate.
 6. A filter devicecomprising: a substrate having at least a first cavity and a secondcavity on a single die; a bonding layer formed on the substrate but notspanning the first cavity or the second cavity; a piezoelectric platebonded to the bonding layer and spanning the first cavity and the secondcavity; wherein the piezoelectric plate is not provided in excessportions that extend a certain length past perimeters of the firstcavity and of the second cavity; a first interdigital transducer (IDT)on a front surface of the piezoelectric plate and having interleavedfingers over the first cavity; a second IDT on a front surface of thefirst piezoelectric plate and having interleaved fingers over the secondcavity; and at least one conductor attaching the first IDT to the secondIDT; wherein the piezoelectric plate is spanning the first cavity andthe second cavity; under the first and second IDTs; and under the atleast one conductor.
 7. The filter device of claim 6, wherein:respective radio frequency signals applied to the first and second IDTsexcite respective primary shear acoustic modes in the piezoelectricplate over the first and second cavities.
 8. The filter device of claim7, wherein a thickness of the piezoelectric plate is selected to tunethe primary shear acoustic modes in the piezoelectric plate.
 9. Thefilter device of claim 6, further comprising connections to the firstand second IDTs that form an RF filter input and output.
 10. A filterdevice comprising: a substrate having a cavity; a piezoelectric plateformed over the substrate and spanning the cavity; wherein the cavityhas a perimeter; and wherein the piezoelectric plate is not provided inexcess portions that extend past the perimeter of the cavity by morethan 5 percent of the perimeter of the cavity; and an interdigitaltransducer (IDT) on a front surface of the piezoelectric plate andhaving interleaved fingers over the cavity.
 11. The filter device ofclaim 10, further comprising: a bonding layer formed on the substrateand bonding the piezoelectric plate to the substrate; wherein bondinglayer does not span the cavity.
 12. The filter device of claim 11,wherein the bonding layer is not provided in excess portions that extenda certain length past the perimeter of the first cavity and of thesecond cavity.
 13. The filter device of claim 10, wherein the substrateis Si, the bonding layer is SiO2, the IDT is metal, and thepiezoelectric plate is one of lithium niobate or lithium tantalate. 14.The filter device of claim 10, wherein a radio frequency signal appliedto the IDT excites a primary shear acoustic mode in the piezoelectricplate over the cavity.
 15. The filter device of claim 14, wherein athickness of the piezoelectric plate is selected to tune the primaryshear acoustic mode in the piezoelectric plate.